HSDPA (High Speed Downlink Packet Access) is one of high-speed packet transmission techniques developed by a standardization organization called 3GPP (Third Generation Partnership Project). It is standardized in “Release 5” of 3GPP and thereafter. HSDPA employs an adaptive modulating system. The adaptive modulating system comprehensively determines states of fluctuating radio wave propagation paths, i.e. determines changes in transmission of radio wave in the air, and automatically selects the best modulation method. Specifically, when the transmission path is in a noisy state, a modulation method is automatically switched to QPSK (Quadrature Phase Shift Keying) that is highly stable but a low speed operation. When the transmission path is in a good state, the modulation method is automatically switched to 16QAM (16 Quadrature Amplitude Modulation) that provides a higher speed operation.
FIG. 1 shows an HSDPA-based communication system. In FIG. 1, a UE (user equipment) 10 transmits a CQI (Channel Quality Indicator) value to a radio base station 11 by using an uplink dedicated control channel HS-DPCCH (High Speed-Dedicated Physical Control Channel) to indicate a radio wave reception state. The radio base station 11 contains an HS-DPCCH receiving section 12, an HS-PDSCH (High Speed-Physical Downlink Shared Channel) transmitting section 13, and a radio base station unit 14. The radio base station unit 14 determines a modulation method, an encoding method, and the like based on the CQI value transmitted from the UE 10. In accordance with the modulation method, the encoding method, and the like, the radio base station 11 transmits packets to the UE 10 by using a downlink shared channel HS-PDSCH. The UE 10 receives the packets sent to itself through the downlink shared channel HS-PDSCH.
In FIG. 1, the radio base station 11 performs scheduling of destination UE for every TTI (Transmission Time Interval), and transmits the packets by using the downlink shared control channel HS-PDSCH. When receiving the packets sent to itself through the downlink shared control channel HS-PDSCH, the UE 10 decodes the packets based on HARQ (Hybrid Automatic Repeat Request). If the packet is decoded properly, the UE 10 transmits an ACK (ACKnowledgement) signal by using the uplink dedicated control channel HS-DPCCH. If decoding of the packet is unsuccessful, the UE 10 transmits a NACK (Negative ACKnowledgement) signal by using the uplink dedicated control channel HS-DPCCH. The radio base station 11 performs a packet retransmission control based on the ACK signal or the NACK signal received from the UE 10. The UE 10 may not be able to return the ACK signal nor the NACK signal, depending on the state of the radio wave. The radio base station 11 takes such a state into account to perform the packet retransmission control.
FIG. 2 shows a block configuration of the radio base station unit 14. In FIG. 2, the radio base station 11 contains an HS-DPCCH decoding section 20, a CQI receiving section 21, a scheduling determining section 22, an ACK/NACK receiving section 23, a BLER (Block Error Rate) calculating section 24, and a resource allocating section 25. The HS-DPCCH decoding section 20 decodes a reception signal received from a HS-DPCCH receiving section 12. The CQI receiving section 21 receives the CQI value as a result of decoding. The ACK/NACK receiving section 23 receives the ACK signal or NACK signal obtained through the decoding. The scheduling determining section 22 refers to the CQI value received by the CQI receiving section 21, and determines in units of TTI, a single UE or a plurality of UEs for transmitting packets according to a predetermined scheduling algorithm. The BLER calculating section 24 calculates a BLER based on a reception result of the ACK signal or the NACK signal received by the ACK/NACK receiving section 23. When the ACK/NACK receiving section 23 has received the ACK signal or the NACK signal, the BLER calculating section 24 can calculate the BLER from the ACK signal and the NACK signal. However, when the ACK/NACK receiving section 23 has not been able to receive the ACK signal nor the NACK signal because of the state of the radio wave, the BLER calculating section 24 can calculate the BLER by using data other than the ACK signal and the NACK signal. The resource allocating section 25 allots communication resources by determining TBS (Transport Block Size) based on the CQI value, determining the number of codes (the number of HS-PDSCHs (High Speed-Physical Downlink Shared Channel), and determining the modulation method, etc.
FIG. 3 is a diagram showing a packet transmitted from the radio base station. FIG. 3 shows a single packet as an example. The entire length of the packet is expressed with TBS in a unit of bits. The packet shown in FIG. 3 has a 21-bit header. The header contains information such as a queue ID, and the length or number of MAC-d PDUs (Medium Access Control-dedicated Protocol Data Unit). The MAC-d PDUs are inserted in the packet. The size of MAC-d PDU is decided to be 336 bits or 656 bits on the standard. Here, 336-bit MAC-d PDUs are illustrated as an example. The remaining section of the packet following the header and the MAC-d PDUs is a padding section.
FIG. 4 shows a CQI mapping table. In the HSDPA, UEs are classified into a plurality of categories, and a CQI mapping table is prepared for each category. The CQI mapping table shown in FIG. 4 is applied to the UE of category “10”. In FIG. 4, numerical values in a “CQI” column indicate the CQI values that are transmitted from the UE by using the uplink dedicated control channel HS-DPCCH. There are thirty values from “1” to “30”. Numerical values in a “TBS” column indicate TBS of the packet to be transmitted to the UE. The TBS can be calculated from the CQI value uniquely. The TBS is not available when the CQI value is “0”. Numerical values in a “code count” column indicate the number of downlink shared channels HS-PDSCH, and a “modulation type” column indicates a modulation method at the time of transmitting the packet. The code count and the modulation type are out of ranges when the CQI value is “0”.
As shown in FIG. 4, under the standardization of 3GPP, thirty values from “1” to “30” are defined as the CQI values. Therefore, there are at most thirty kinds of TBSs corresponding to the CQI values for each category of the UE. In this example, the numbers of MAC-d PDUs contained in TBS does not necessarily become continuous. This tendency is especially prominent when the CQI values are high in the UE of category “10”. For example, referring to the CQI values of “24”-“30” in FIG. 4, the TBSs in those sections are “11418 (CQI=24)”, “14411 (CQI=25)”, “17237 (CQI=26)”, “21754 (CQI=27)”, “23370 (CQI=28)”, “24222 (CQI=29)”, and “25558 (CQI=30)”, respectively. The numbers of MAC-d PDUs are “33 (CQI=24)”, “42 (CQI=25)”, “51 (CQI=26)”, “64 (CQI=27)”, “69 (CQI=28)”, “72 (CQI 29)”, and “76 (CQI=30)”, provided that the size thereof is in units of 336 bits as shown in 3GPP TS25. 214, 3GPP TS25. 308, 3GPP TS25. 321 and 3GPP TS25. 858.